Exhaust washed structure and associated composite structure and method of fabrication

ABSTRACT

A composite structure and an associated exhaust washed structure are provided which may be formed of ceramic matrix composite (CMC) materials. A method of fabricating a composite structure which may include the CMC material is also provided. A composite structure may include a corrugated septum extending in a lengthwise direction. The composite structure may also include a flute within which the corrugated septum is disposed to form, for example, a partitioned flute assembly.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/106,512, filed Apr. 21, 2008 which is hereby incorporated herein inits entirety by reference.

FIELD

Embodiments of the present disclosure relate generally to compositestructures and associated methods of fabrication and, more particularly,to composite structures, including a variety of hot exhaust washedstructures, manufactured from ceramic matrix composite (CMC) materialsas well as associated methods of fabrication.

BACKGROUND

A number of exhaust system components of conventional jet engines aswell as other hot exhaust washed structures are fabricated from titaniumalloys. While titanium alloys have a number of advantageous materialproperties, the exhaust system temperatures of next generation jetengines are anticipated to reach a level at which components fabricatedfrom titanium alloys may have an unsatisfactory service life. In thisregard, the historical trend has been for each generation of jet engineto exhaust gasses having greater temperatures than the prior generationin an effort by the engine designers to achieve greater thermodynamicefficiency. However, at the exhaust system temperatures predicted forthe next generation of jet engines, such as temperatures in excess of1,000° F., exhaust system components fabricated of titanium alloys, suchas exhaust system nozzles and exhaust system centerbodies as well asother hot exhaust washed structures, may oxidize relatively rapidly,thereby disadvantageously reducing the service life of the components.

A number of conventional exhaust system components, such as exhaustsystem nozzles and exhaust system centerbodies, have been constructed inthe form of a honeycomb core sandwich. In this regard, these exhaustsystem components can include a pair of titanium alloy face sheetsdisposed on opposite sides of a honeycomb core, which may also be formedof a titanium alloy. In order to reduce the noise emanating from anengine, some of the exhaust system components may include Helmholtzresonators. In order to provide Helmholtz resonators, perforations orother holes may be defined, such as by drilling, through the titaniumalloy face sheet which is adjacent to the high-speed flow of exhaustgasses. The perforations or other holes defined by the titanium alloyface sheet open into respective cells of the honeycomb core. Byappropriately tuning the geometry of the honeycomb cells, the noiseemanating from the engine may be advantageously reduced.

In an effort to provide exhaust system components and other hot exhaustwashed structures that can withstand higher temperatures, such astemperatures in excess of 1,000° F., components comprised ofhigh-temperature metal alloys have been proposed. However, thesehigh-temperature metal alloys, such as Inconel 718, Rene 41 andColumbium alloys, are undesirably heavy relative to comparablecomponents fabricated from titanium alloys. Since the weight of anaircraft, including its engine, is a key concern relating to both theperformance and cost of operation of the aircraft, the use of exhaustsystem components and other hot exhaust washed structures formed ofhigh-temperature metal alloys that are heavier than correspondingtitanium alloy components have not proven to be a desired solution.

Accordingly, it would be desirable to provide exhaust system components,such as nozzles and centerbodies, as well as other hot exhaust washedstructures which can withstand exhaust gas temperatures in excess of1,000° F. without any meaningful reduction of the service life of thecomponents. Additionally, it would be advantageous to provide exhaustsystem components, such as nozzles and centerbodies, and other hotexhaust washed structures which can withstand such higher exhaust gastemperatures, but which weigh no more than corresponding titanium alloycomponents so as to not increase the weight of the engine.

SUMMARY

Embodiments of the present disclosure therefore provide a compositestructure and an associated exhaust washed structure which may includeceramic matrix composite (CMC) materials, thereby permitting thecomposite structure to withstand temperatures in excess of 1,000° F.,such as those potentially generated by the exhaust gasses of the nextgeneration of aircraft engines, without a meaningful reduction in theservice life of the composite structure and without increasing theweight of the exhaust washed structures relative to correspondingtitanium alloy components. According to other embodiments of the presentdisclosure, methods of fabricating a composite structure which may alsoinclude the CMC material are also provided, thereby permitting engineexhaust system components and other hot exhaust washed structures to befabricated, such as from CMC material, so as to be capable ofwithstanding temperatures in excess of 1,000° F.

In one embodiment, a composite structure is provided which includes acorrugated septum, which may be comprised of a CMC material, extendingin a lengthwise direction. The composite structure also includes aflute, which may also be formed of CMC material, having the corrugatedseptum disposed therein. The corrugated septum and the flute define apartitioned flute assembly, such as a partitioned CMC flute assembly.The resulting partitioned CMC flute assembly may have radiused cornerportions when taken in lateral cross-section. In one embodiment, thecomposite structure includes first and second face sheets and aplurality of partitioned CMC flute assemblies disposed between the facesheets.

The composite structure of one embodiment also includes a bulk acousticabsorber disposed proximate the corrugated septum and within the flute.The bulk acoustic absorber may be interspersed with convolutes of thecorrugated septum. The bulk acoustic absorber may be formed of a ceramicmaterial.

In accordance with another embodiment, an exhaust washed structure isprovided which includes a wall member and a plurality of partitionedflute assemblies positioned upon the wall member. Each partitioned fluteassembly extends lengthwise along the wall member. Additionally, eachpartitioned flute assembly is positioned laterally adjacent anotherpartitioned flute assembly. Further, each partitioned flute assemblyincludes a corrugated septum, which may be formed of a CMC material,extending in the lengthwise direction and a flute, which may also beformed of CMC material, in which the corrugated septum is disposed.

The wall member of one embodiment defines a plurality of sections spacedlongitudinally therealong. In this embodiment, a plurality ofpartitioned CMC flute assemblies are positioned upon each section of thewall member. Each partitioned CMC flute assembly may extend lengthwisealong the respective section of the wall member. Additionally, eachpartitioned CMC flute assembly may be positioned laterally adjacentanother partitioned CMC flute assembly within the respective section ofthe wall member. In this regard, each partitioned CMC flute assembly hasa height that may change in a longitudinal direction to facilitateside-by-side positioning of the partitioned CMC flute assemblies.

As noted above, each partitioned CMC flute assembly may also include abulk acoustic absorber which may be formed of a ceramic material. Thebulk acoustic absorber is disposed proximate the corrugated septum andwithin the flute of CMC material. In one embodiment, the bulk acousticabsorber is interspersed with convolutes of the corrugated septum. Theexhaust washed structure of this embodiment may also include first andsecond face sheets disposed on opposite sides of the plurality ofpartitioned CMC flute assemblies. Each partitioned CMC flute assemblymay include radiused corner portions in lateral cross-section.

A method of fabricating a composite structure is also provided inaccordance with another embodiment of the present disclosure. The methodof this embodiment provides a corrugated septum, which may be formed ofa CMC material, that extends in a lengthwise direction. The corrugatedseptum is cured and then disposed within a flute, which may also beformed of CMC material. The corrugated septum may then be bonded withinthe flute to form a partitioned flute assembly, such as a partitionedCMC flute assembly.

A bulk acoustic absorber may be positioned proximate the corrugatedseptum, such as by interspersing the bulk acoustic absorber withconvolutes of the corrugated septum. As such, the method of oneembodiment may also include the disposition of a plurality ofpartitioned CMC flute assemblies between first and second face sheets tofabricate, for example, an exhaust washed structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus provided a description in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is an illustration of a perspective view of a corrugated septumformed of a ceramic matrix composite (CMC) material and a plurality ofrigidized absorber blocks positioned relative to the corrugated septumin accordance with one embodiment of the present disclosure;

FIG. 2 is an illustration of a perspective view of a composite structurein accordance with one embodiment of the present disclosure;

FIG. 3 is an illustration of a perspective view of a composite structureincluding a plurality of partitioned CMC flute assemblies in accordancewith one embodiment of the present disclosure;

FIG. 4 is an illustration of an engine exhaust system nozzle having aplurality of partitioned CMC flute assemblies disposed upon an innerskin in accordance with one embodiment of the present disclosure;

FIG. 5 is an illustration of a perspective view of an exhaust systemnozzle, as shown in FIG. 4, including a complete set of partitioned CMCflute assemblies prior to placement of an outer skin thereover inaccordance with one embodiment of the present disclosure;

FIG. 6 is an illustration of a perspective view of a centerbodyincluding a plurality of sections with a plurality of partitioned CMCflute assemblies disposed side-by-side within each section in accordancewith one embodiment of the present disclosure; and

FIG. 7 is an illustration of a flow chart of a method of fabricating acomposite structure in accordance with one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure now will be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments are shown. Indeed, these embodiments may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

As described hereinbelow, a composite structure 10 is provided which maybe utilized in a wide variety of applications. As a result of itsability to withstand relatively high temperatures, such as in excess of1,000° F., without any meaningful reduction of its service life, thecomposite structure 10 is particularly useful in high-temperatureapplications. Additionally, the composite structure 10 of someembodiments of the present disclosure may be designed to provideimproved structural and acoustic performance relative to moreconventional structures formed of metallic alloys. Further, thecomposite structure 10 of some embodiments of the present disclosure mayalso be superior in terms of damage tolerance to some other sandwich CMCconstructions, such as open fluted core constructions. As such, thecomposite structure 10 of one embodiment can be utilized to form variousengine exhaust system components, such as exhaust system nozzles and/orcenterbodies, for aircraft engines as well as other hot exhaust washedstructures, such as aft fairing heat shields and thrust reverser innerwalls, or the like.

As shown in FIG. 1, a composite structure 10 in accordance with oneembodiment to the present disclosure includes a corrugated septum 12which may be formed of a ceramic matrix composite (CMC) material. Asknown to those skilled in the art, CMC material is a reinforced ceramicmaterial created from substantially continuous fibers bound in a ceramicmatrix. The fibers can be in tape or cloth form and may include, but arenot limited to, fibers formed from silicon carbide, alumina,aluminosilicate, aluminoborosilicate, carbon, silicon nitride, siliconboride, silicon boronitride, and similar materials. The ceramic matrixmay include, but is not limited to, matrices formed fromaluminosilicate, alumina, silicon carbide, silicon nitride, carbon, andsimilar materials. In one embodiment, the CMC material is comprised ofalumina fibers in an aluminosilicate matrix, i.e., an Oxide/Oxide CMC.In another embodiment, the CMC material may be comprised of siliconcarbide fibers in a silicon carbide matrix, i.e., an SiC/SiC CMC.

As noted, the corrugated septum 12 is corrugated so as to define arelatively sinuous pattern that extends in a longitudinal or lengthwisedirection 13. While the corrugated septum 12 may be formed of two ormore corrugated sections that are positioned end to end, the corrugatedseptum 12 of one embodiment extends continuously in the longitudinaldirection 13 from one end of the composite structure 10 to the otherend. While the corrugated septum 12 can define a truly sinuous shape,the corrugated septum 12 of the illustrated embodiment is comprised of aplurality of linear segments joined to one another via generally planarwebs proximate opposed lateral sides of the composite structure 10. Thecorrugated septum 12 of the illustrated embodiment therefore defines anumber of alternately facing, truncated v-shaped sections or convolutes12 a joined to one another and extending in the longitudinal direction13. Once the corrugated septum 12 has been formed into the desiredcorrugated shape, such as by placement upon a female tool, such aswithout limitation an aluminum or steel tool, the corrugated septum 12is cured. See operations 42 and 44 of the exemplary method 40 of FIG. 7.In this regard, while no particular cure process is required,oxide/oxide CMC structures are generally cured in a two-step process.The initial cure, made at temperatures on the order of 350° F.,strengthens the structure sufficiently that it can be removed from thelayup tool. The second, sintering, step of the processing is made at ahigher temperature, such as 500° F. to 2,200° F., and may be made withthe structure either free standing or partially supported.

The composite structure 10 may also include a bulk acoustic absorber 14disposed proximate the corrugated septum 12. See operation 46 of FIG. 7.Not all composite structures need a bulk acoustic absorber 14, but sincethe bulk acoustic absorber 14 is generally designed to absorb noise,composite structures in accordance with embodiments of the presentdisclosure that are designed for sound reduction purposes generallyinclude a bulk acoustic absorber 14 as shown and described hereinafter.The bulk acoustic absorber 14 may be formed of various materialsincluding, without limitation, fibrous ceramic material. The bulkacoustic absorber 14 is advantageously formed of a material that iscapable of withstanding the temperatures employed during construction ofthe composite structure 10. Additionally, the bulk acoustic absorber 14may be designed to efficiently dissipate acoustic energy, such as byconverting the acoustic energy to waste heat. For example, the bulkacoustic absorber 14 may include structures, such as without limitationcantilever beams and/or intermittently supported beams or plates, whichresonate at frequencies excited by the acoustic vibrations. The bulkacoustic absorber 14 may therefore be formed of felts, loosely wovenmaterials and/or foams which can include the foregoing resonatingstructures. The bulk acoustic absorber 14 may be non-rigid during use ofthe composite structure in order to appropriately absorb noise. For somemanufacturing processes, however, it may be advantageous to temporarilyrigidize the bulk acoustic absorber 14 as depicted in operation 46 ofFIG. 7 by infusing the bulk acoustic absorber 14 with a fugitive matrixmaterial, such as an organic material including, for example, plasticmaterials as known to those skilled in the art. In the illustratedembodiment, for example, the rigidized bulk. acoustic absorber 14 mayinclude a plurality of rigidized blocks forming the bulk acousticabsorber 14 interspersed with the corrugated septum 12. As shown in FIG.1, for example, the rigidized blocks forming the bulk acoustic absorber14 may be sized and shaped to fit snugly within each corrugation of thecorrugated septum 12. The rigidized blocks forming the bulk acousticabsorber 14 may be bonded to the corrugated septum 12 to form asubassembly as shown in FIG. 1 and in operation 46 of FIG. 7.Thereafter, during the sintering step, the fugitive material sublimatesand escapes through openings in the structure or through the porous CMCshells. Generally, the bulk acoustic absorber 14 must be non-rigid oncethe composite structure 10 is in use since the rigidized absorber maynot effectively reduce the noise.

In addition to the corrugated septum 12, a flute 16 is also formed, asshown in FIG. 2 and in operation 48 of FIG. 7. A flute 16 is generally atubular core member of a sandwich structure. In accordance withembodiments of the present disclosure, one or more flutes 16 may beformed of CMC material, such as Oxide/Oxide or SiC/SiC, and, as such,are termed CMC flutes 16 herein by way of example, but not oflimitation. Each CMC flute 16 may be formed upon a mandrel, such aswithout limitation an aluminum mandrel. The CMC flute 16 is then vacuumbagged and cured, such as by means of the two-step process describedabove. Thereafter, the mandrel is removed from the CMC flute 16 and acorrugated septum 12 is inserted and bonded within the CMC flute 16, asprovided by operation 50 of FIG. 7. In one embodiment, the corrugatedseptum 12 when cured is wider than an individual CMC flute 16 such thatthe corrugated septum 12 may be cut into lengthwise strips after beingcured with each strip being inserted and bonded into a respective CMCflute 16.

The combination of the corrugated septum 12 and the flute 16, such asillustrated in FIG. 2, may be termed a partitioned flute assembly 15. Asa result of the construction of the corrugated septum 12 and the flutes16 from a CMC material in accordance with one embodiment, thepartitioned flute assembly 15 is referenced herein as a partitioned CMCflute assembly 15 by way of example, but not of limitation. CMC flutes16 may be formed to have a number of different shapes in lateralcross-section. For example, the CMC flutes 16 may be formed to have atrapezoidal shape in lateral cross-sections. Alternatively, CMC flutes16 may be formed to have a more rectangular shape in lateralcross-section, as shown in FIG. 2. As shown in FIG. 2, for example, thepartitioned CMC flute assembly 15 may have radiused corners 15 a.

A composite structure 10 of embodiments of the present disclosureadvantageously concurrently addresses structural, acoustic and damagetolerance issues, while being capable of deployment in high temperatureenvironments, such as those characteristic of jet engine exhaustsystems, without any meaningful reduction in its service life. In thisregard, the extension of the corrugated septum 12 in a longitudinaldirection 13 in combination with the rigidized form of the bulk acousticabsorber 14 reduces noise propagation both in the longitudinal direction13 and through the composite structure 10 in any lateral direction. As aresult of its elongate configuration, the composite structure 10 ofembodiments of the present disclosure has substantial strength andstiffness in the longitudinal direction 13. Moreover, by forming thecorrugated septum 12 and the surrounding plies of the CMC flute 16 froma CMC material, the composite structure 10 also has substantial strengthand stiffness in lateral directions substantially perpendicular to thelongitudinal axis 13. Additionally, the corrugated septum 12 may provideimproved damage tolerance. For example, the corrugated septum 12 mayinsure or at least increase the likelihood that an object penetratingone of the lateral sides of the composite structure 10 would losesignificant energy while passing through the composite structure 10. Inthis regard, the damage tolerance of the composite structure 10 may betuned by more closely spacing the convolutes 12 a of the corrugatedseptum 12 and/or by increasing the thickness of the corrugated septum 12in order to increase the damage tolerance or by more widely separatingthe convolutes 12 a and/or by decreasing the thickness of the corrugatedseptum 12 in order to reduce the damage tolerance. As such, thecomposite structure 10 provides advantageous structural, acoustic anddamage tolerance characteristics. For an exhaust system application asdescribed below in conjunction with FIGS. 3-6, it may be advantageous torestrict impact damage to a single face sheet 18 and the underlying coreformed of the composite structure 10, for reasons regarding bothaerodynamic performance and repairability. If only one face sheet 18 ispenetrated, the structure 10 is still capable of separating two exhaustflows. Also, damage to a single face sheet 18 is easier to repair in thefield than damage penetrating both face sheets 18. If only one facesheet 18 a has been penetrated, the back face sheet 18 b can be used tocontain variously reinforced slurry mixtures that can then be cured inplace sufficiently that the part can be returned to service. If thedamage penetrates both face sheets 18, the part may need rework. Forthis reason, a fine celled core is better from a damage tolerancestandpoint than a more open core structure. Similarly, a thick walledcore is more likely to stop a penetrator short than a thin walled core.The desire for damage tolerance is balanced against the desire tominimize weight in flight structures. The corrugated septum 12 ofembodiments of the present disclosure allows tailoring damage toleranceversus weight to obtain the optimal core design for a given application.

Moreover, the composite structure 10 has a design that lends itself tobeing manufacturable. In this regard, the corrugated septum 12 formed ofa CMC material may be appropriately shaped. The corrugated septum 12 maybe cured on a layup tool, such as the type of tool that would be usedfor conventional polymeric matrix composite layup, for example andwithout limitation, a block of steel with the convolute shape machinedtherein. Since draping the CMC prepreg over the convolutes 12 a by handcould be time consuming, a manufacturing aid could be used topre-configure the CMC prepreg before placing it on the tool. In oneembodiment, a manufacturing aid could be a pair of combs, that is,parallel pegs mounted in long bases at the spacing of the desired septumcorrugations. The combs would, in turn, be mounted in a frame whichguided and controlled the extent of their relative movement. In use, along strip of prepreg material would be placed between the combs and thecombs would be moved so that the pegs of one comb passed between thepegs of the other comb, forming the prepreg fabric into the correctshape to drape smoothly over the layup tool. The corrugated septum 12can then be cured as described above. A bulk acoustic absorber 14 maythen be inserted between individual corrugations of the corrugatedseptum 12 in either a rigidized or non-rigidized form. In oneembodiment, the corrugated septum 12 and the bulk acoustic absorber 14may thereafter be inserted into and bonded within a CMC flute 16 whichhas been formed and cured as described above, thereby forming apartitioned CMC flute assembly 15. Alternatively, the flute 16 may bewrapped in-place about a pre-cured corrugated septum 12 that is at leastpartially filled with a fugitive tooling material which may or may notinclude a bulk acoustic absorber 14 depending upon the desire for theresulting composite structure 10 to provide acoustic attenuation. Whilecertain construction methods have been herein described, the compositestructure 10 of embodiments of the present disclosure can bemanufactured in a wide variety of manners.

Once the composite structure 10 is formed and cured, one or more of theprecured composite structures 10 are generally positioned between a pairof face sheets 18 to form a resulting structural assembly 19, such asshown in FIGS. 3-6 and in operation 52 of FIG. 7 and described below. Ifdesired, fillets or noodles 17, as shown in FIG. 3, also generallyformed of a CMC material, may be positioned or inserted at the nodes orcorners between the partitioned CMC flute assemblies 15. The face sheets18 can be comprised of a variety of different materials, but also arecomprised of a CMC material in one embodiment, such as the same CMCmaterial forming the corrugated septum 12 and the CMC flute 16. Thestructural assembly 19 is then vacuum bagged and cured in an autoclavewith pressures up to 100 psi and temperatures up to 400° F. Then, thestructural assembly 19 is removed from the autoclave and vacuum bag andthen sintered without pressure at elevated temperatures using a steppedprofile that may range from 500° F. to 2,200° F. The composite structure10 may be formed to have a rectangular shape as shown in FIG. 2, or atrapezoidal shape. As will be apparent to those skilled in the art, thecomposite structure 10 can have other shapes depending upon theapplication in which the composite structure 10 would be deployed.Moreover, the partitioned CMC flute assemblies 15 can be packed adjacentto one another or placed separately at intervals between the face sheets18. Embodiments therefore provide a structural assembly 19 in which theperformance of longitudinally extending CMC flutes 16 is enhanced by theaddition of a corrugated septum 12. The resulting structural assembly 19is a shear carrying assemblage of tubular partitioned CMC fluteassemblies 15, typically extending in a substantially parallel fashion.The flute 16 cross-section can vary in its size along its length toaccommodate the space between face sheets 18. The flute 16 is typicallythe primary load carrying core member. A fundamental advantage ofpartitioned CMC flute assemblies 15 is that they provide a large surfacefor bonding to the sandwich face sheets 18. Competing core options, suchas honeycomb, require edge bonding, which may be difficult to accomplishin CMC materials. As noted above, the corrugated septum 12 may performat least three functions: it may block acoustic transmission down thelength of the flute 16, it may stiffen the flute 16 laterally and, byresisting though-the-thickness penetration of the sandwich, it mayimprove damage tolerance. Moreover, for manufacturability, thecorrugated septum 12 may be formed from a single corrugated strip of CMCmaterial.

Although composite structures 10 of embodiments of the presentdisclosure may be deployed in a variety of applications, the compositestructure 10 of one embodiment forms a portion of an engine exhaustsystem component or other hot exhaust washed structure 20, as shown inFIGS. 4-6. For example, the composite structure 10 of one embodiment mayform a portion of an exhaust washed structure 20, such as an exhaustsystem nozzle. As shown in FIGS. 4 and 5, the exhaust washed structure20 may be an engine component having a wall member 22, e.g., an innerface sheet 18 a (FIG. 3), extending from a forward end 22 a to anopposed rearward end 22 b. A plurality of composite structures 10 maythen be positioned side by side about the exhaust washed structure 20such that each composite structure 10 also extends from a forward end 10a proximate the forward end 22 a of the exhaust washed structure 20 to arearward end 10 b proximate the rearward end 22 b of the exhaust washedstructure 20. The composite structures 10 which are utilized in thefabrication of an exhaust washed structure 20 of the embodiment shown inFIGS. 4 and 5 are not rectangular solids, but instead, have a moretapered shape, such as shown in FIG. 1. In other words, when consideredin a lengthwise extending direction 13 from the forward end 22 a to therearward end 22 b, the height of the composite structure 10 initiallyincreases to a maximum height 21 before gradually decreasing thereafterin height to the rearward end 22 b. The composite structures 10 may haveother shapes in other applications, if so desired. The compositestructures 10 are positioned side by side so as to extendcircumferentially about the entire exhaust washed structure 20. Once thecomposite structures 10 have been positioned upon the wall member 22,another face sheet 18 b (FIG. 3), e.g., an outer skin, may be disposedupon the composite structures 10. The inner 18 a and outer 18 b facesheets may also be formed of CMC materials, e.g., oxide/oxide CMC orSiC/SiC CMC, with the inner 18 a and outer 18 b face sheets being curedafter the outer face sheet 18 b is placed thereupon, as described abovein conjunction with FIG. 4.

In order to reduce the noise emanating from the engine, one or moreperforations, e.g. holes, may be formed through the wall member 22 andthrough the CMC flutes 16 of the composite structure 10. As such, theairflow through the exhaust washed structure 20, e.g., a high speed flowof exhaust gases, will be in fluid communication with the interior ofthe composite structure 10, namely, the corrugated septum 12 and thebulk acoustic absorber 14, which serves to dissipate the noiseassociated with the airflow through the exhaust washed structure 20.

In another example, the composite structures 10 may be utilized in theconstruction of a centerbody 30. As shown in FIG. 6, a centerbody 30 ofan engine exhaust system may have a frustoconical shape. As a result ofthe frustoconical shape, composite structures 10 positioned upon thecenterbody 30 would desirably be tapered from the forward end 10 a tothe rearward end 10 b (FIG. 4) in order to be positioned immediatelyadjacent to each other. In order to reduce or eliminate the taper and toaccordingly simplify the manufacturing process, while insuring that thecomposite structures 10 can be positioned close to, if not, immediatelyadjacent one another in order to take advantage of the structural,acoustical and damage tolerance properties of the composite structures10, the centerbody 30 may be divided into a plurality of sections 32 orbays. Each section 32 extends circumferentially about the centerbody 30and defines a different longitudinal portion of the centerbody 30.

Within each section 32, a plurality of composite structures 10 may bepositioned in a side by side manner upon a wall member 34, e.g., aninner face sheet 18 a (FIG. 3), of the centerbody 30 such that eachcomposite structure 10 extends from a forward end 10 a proximate aforward end 32 a of the respective section 32 to a rearward end 10 bproximate a rearward end 32 b of the respective section 32. By havingdivided the centerbody 30 into sections 32, the tapering of thecomposite structures 10 in a lengthwise extending direction can bereduced or eliminated since the composite structures 10 need not extendalong the entire length or even a majority of the length of thecenterbody 30. Thus, the composite structures 10 may be fabricated in arelatively straightforward manner and may still be positioned closely,if not immediately adjacent, to one another so as to provide sufficientstructural, acoustic and damage tolerance performance. As with theexhaust washed structure 20 of FIG. 5, an outer face sheet 18 b (FIG. 3)may be disposed over the composite structures 10 to complete thefabrication of the centerbody 30. As with the exhaust washed structure20 of FIG. 5, the inner 18 a and outer 18 b face sheets are also formedof CMC materials, such as oxide/oxide CMC or SiC/SiC CMC.

Many modifications and other embodiments of the embodiments set forthherein will come to mind to one skilled in the art to which theseembodiments pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the disclosure is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims. Forexample, although certain fabrication techniques were described, thecomposite structure 10 may be formed in other manners such as afabrication method in which flute halves are formed and partially curedin separate tooling before insertion of the corrugated septum 12.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A method of fabricating an exhaust washedcomposite structure comprising: providing a corrugated septum comprisedof a ceramic matrix composite (CMC) material extending in a lengthwisedirection; curing the corrugated septum; positioning a bulk acousticabsorber so as to be interspersed with convolutes of the corrugatedseptum; forming a flute of CMC material; and disposing the corrugatedseptum following curing and the bulk acoustic absorber within the fluteto form a partitioned flute assembly.
 2. A method according to claim 1wherein positioning the bulk acoustic absorber comprises rigidizing thebulk acoustic absorber and bonding rigidized blocks of the bulk acousticabsorber to the corrugated septum.
 3. A method according to claim 1further comprising disposing a plurality of partitioned flute assembliesbetween first and second face sheets.
 4. A method according to claim 1further comprising bonding the corrugated septum within the flute.
 5. Amethod according to claim 1 wherein disposing the corrugated septumwithin the flute forms a partitioned CMC flute assembly.
 6. A methodaccording to claim 5 wherein the partitioned CMC flute assembly hasradiused corner portions.
 7. A method according to claim 1 furthercomprising providing the bulk acoustic absorber comprised of a non-rigidmaterial.
 9. A method according to claim 1 further comprising providingthe bulk acoustic absorber comprised of a ceramic material.
 9. A methodaccording to claim 1 wherein forming the flute comprises forming theflute so as to define at least one perforation to establish fluidcommunication between the corrugated septum and the bulk acousticabsorber and an external environment.
 10. A method of fabricating anexhaust washed structure comprising: providing a wall member; andpositioning a plurality of partitioned flute assemblies upon the wallmember and laterally adjacent another partitioned flute assembly so thateach partitioned flute assembly extends lengthwise along the wallmember, wherein each partitioned flute assembly comprises: a corrugatedseptum comprised of a ceramic matrix composite (CMC) material extendinglengthwise; and a flute of CMC material having the corrugated septumdisposed therein.
 11. A method according to claim 10 wherein the wallmember defines a plurality of sections spaced lengthwise therealong,wherein positioning the plurality of partitioned flute assemblies uponthe wall member comprises positioning a plurality of partitioned fluteassemblies upon each section of the wall member so that each partitionedflute assembly extends lengthwise along the respective section of thewall member, and wherein each partitioned flute assembly is positionedlaterally adjacent another partitioned flute assembly within therespective section of the wall member.
 12. A method according to claim10 further comprising providing the plurality of partitioned fluteassemblies with each partitioned flute assembly changing in lateralwidth in a lengthwise direction.
 13. A method according to claim 10further comprising providing the plurality of partitioned fluteassemblies with each partitioned flute assembly further comprising abulk acoustic absorber disposed proximate the corrugated septum andwithin the respective flute.
 14. A method according to claim 13 whereinthe bulk acoustic absorber is interspersed with convolutes of thecorrugated septum.
 15. A method according to claim 13 wherein the bulkacoustic absorber is comprised of a non-rigid material.
 16. A methodaccording to claim 13 wherein the bulk acoustic absorber is comprised ofa ceramic material.
 17. A method according to claim 10 furthercomprising providing the plurality of partitioned flute assemblies witheach partitioned flute assembly in lateral cross-section having radiusedcorner portions.
 18. A method according to claim 10 further comprisingestablishing fluid communication between the corrugated septum and theexternal environment via at least one perforation defined by the wallmember and the flute.